This project involves improving the performance of standard fluorometric equipment for use in the assay of small amounts of enzymes, in particular ATPase enzymes. Modifications consist of a temperature controlled cuvette holder and a new kind of fluorescent lamp intensity regulator. The system was evaluated by measuring the ATPase activity of isolated segments of rabbit kidney tubule. Sensitivity and stability of the system were sufficient to measure 10 to the minus 13th power/mole ATP hydrolyzed per minute. In a total sample volume of 100 micro liters.